Aircraft have used on-board inert gas generating systems (OBIGGS) to protect against fuel tank explosions by replacing the potentially explosive fuel vapor/air mixture above the fuel in the ullage space of the tanks with nitrogen-enriched air (NEA). The NEA is generated by separating oxygen from local, ambient air and pumping the inert, oxygen impoverished NEA into the tanks.
Production of NEA typically is carried out by means of an apparatus relying on permeable membranes, or else on molecular sieves. The air separation apparatus is generally referred to as an air separation module (ASM). A selectively permeable membrane ASM typically comprises a bundle of hollow fiber membranes packaged in a cylindrical shell with an inlet and outlet at the ends of the shell, and a shell side vent port. When pressurized air enters the ASM shell and passes into the hollow fibers, both oxygen and nitrogen are separated from the air stream due to permeation through the fiber walls. However, since the fiber walls are more permeable to oxygen than nitrogen, the non-permeating gas stream becomes oxygen deficient and nitrogen enriched, while the permeating gas stream is oxygen enriched and nitrogen deficient. The oxygen enriched air (OEA) exits through the side vent port and can be recaptured, but often the OEA is considered a waste gas that is exhausted overboard. The remaining NEA flows out of the ASM via the outlet port and is distributed to the ullage space of the fuel tank or tanks for the purpose of inerting the fuel tanks and thereby reducing flammability. The ASM operates more efficiently, in terms of permeability of oxygen as the membrane temperature increases. The purity of the NEA is also dependent upon the pressure differential and pressure ratio across the hollow fiber membranes, as well as the NEA flow rate and membrane temperature.
In many if not most commercial airplane applications, pressurized air used for NEA generation will originate from either an engine bleed or from a cabin air pressure source. With an engine bleed pressure supply, compressed hot air is ducted from an engine bleed air supply line and then cooled by a heat exchanger to an optimal temperature for maximum ASM performance. This use of engine bleed air can decrease engine performance and can lead to increased fuel consumption. Accordingly, it is desirable to limit the amount of engine bleed air that is needed during the various segments of the aircraft flight profile and particularly during cruise.
The nitrogen that must be generated and sent to the fuel tank to maintain inertness varies greatly during a flight. During climb, the ambient pressure decreases as altitude increases. As a result, ullage gas in the fuel tank may be vented overboard to maintain pressure equilibrium or a specified pressure differential between the tank and the outside environment. During this phase of the flight, the amount of nitrogen required to maintain an inert condition within the fuel tank is relatively low. Likewise, during the cruise regime, altitude is held relatively constant and the amount of nitrogen required to maintain an inert condition is relatively low.
As an aircraft descends, the ambient pressure increases as the altitude decreases. Consequently, there is typically a large inrush of outside air into the ullage space during the descent regime. This is especially true for an airplane with an open fuel tank vent, which most commercial airplanes have. The inrush of atmospheric air at 21% oxygen by volume, can quickly raise the oxygen concentration in the ullage, thereby spoiling an inert tank. Thus, there is a high demand upon the inerting system to supply a flow of nitrogen to the fuel tank during the descent regime. In fact, much of the inerting system capacity is required only during descent.
The inerting of an airplane fuel tank thus presents a significant design challenge to provide an adequate level of inerting capacity at the lowest penalty to the airplane. The penalty to the airplane comes in the form of inerting system weight, parasitic losses and cooling losses. Each of these three elements requires the airplane to burn more fuel and/or carry less payload.
In a known OBIGGS architecture, plural ASMs are provided to allow for low NEA flow from a primary ASM during the cruise phase of an aircraft flight profile and high NEA flow from both the primary and one or more additional secondary ASMs during aircraft descent. During cruise, the primary ASM receives a steady flow of pressurized air at a controlled temperature, thereby maintaining the primary ASM at a desired operating temperature for providing a high purity NEA. The secondary ASMs, however, are idle.
Historically, OBIGGS have been implemented either with all ASMs operating through a single, flow control orifice during all phases of flight, or using a two-flow system with one or more ASMs operating in one flow mode for climb and cruise, and another flow mode for descent. While the two-flow system represents a significant improvement in overall system performance versus the single flow system, it still leaves a significant amount of available system performance untapped.